This invention relates to bipolar transistors and, more particularly, to methods for fabricating bipolar transistors.
Integrated circuits based on complementary metal-oxide-semiconductor (CMOS) transistors contain complementary n-channel and p-channel metal-oxide-semiconductor (MOS) field-effect transistors. CMOS technology allows circuits to be formed that exhibit low power consumption and high levels of integration.
It is sometimes desirable to form another type of transistor such as the bipolar junction transistor (BJT) on the same silicon substrate as the MOS transistors on a CMOS integrated circuit. Bipolar junction transistors can be used to form circuits that are difficult or impractical to form using MOS transistors. Bipolar transistors have three terminals—an emitter terminal, a collector terminal, and a base terminal. Current passes between the emitter and collector under the control of signals applied to the base.
One way of forming a PNP bipolar transistor involves forming an N-well that is connected to the base terminal, a P+ region in the N-well that is connected to the emitter terminal, and a P-well that is adjacent to the N-well and that is connected to the collector terminal. The P+ emitter region may be epitaxially grown in a trench etched within the N-well by selectively depositing silicon germanium material in the trench (i.e., silicon germanium material is grown in direct contact with the N-well). This type of junction in which P+ silicon germanium is grown in direct contact with the N-well is sometimes referred to as a “hetero” junction.
The current that passes from the emitter to the collector in a PNP bipolar transistor may vary exponentially with the voltage difference across the emitter and the base (VBE). It is therefore generally desirable for VBE to be relatively constant during normal operation of the bipolar transistor so as to draw a constant amount of current. The formation of hetero junctions in bipolar transistors may, however, result in substantial VBE variation and can significantly degrade device performance and reliability.